Self-emitting element, display panel, display apparatus, and method of manufacturing self-emitting element

ABSTRACT

A display panel includes a light-emitting layer, a protective layer, a reflective layer, and a reflective surface. The protective layer is deposited on an emitting side of the light-emitting layer and forms an interface with an external medium. The protective layer has a thickness that allows the light emitted from the light-emitting layer to undergo total reflection at least once at the interface in an area of the light-emitting layer. The reflective layer is deposited on an opposite side of the protective layer with respect to the light-emitting layer. The reflective surface is at a periphery of the light-emitting layer and changes the direction of the light propagating inside the protective layer emitted from the light-emitting layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of application Ser. No. 10/718,676 filed Nov. 24,2003. The disclosure of the prior application is hereby incorporated byreference herein in its entirety.

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 2002-365522 filed on Dec. 17,2002 and No. 2003-049396 filed on Feb. 26, 2003; the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

1) Field of the Invention

The present invention relates to a self-emitting element like an organicelectro-luminescent element (hereinafter, “organic EL element”), adisplay panel, a display apparatus, and a method of manufacturing theself-emitting element.

2) Description of the Related Art

In recent years, self-emitting flat panel displays (hereinafter, “FPD”)in which display panels including light-emitting elements (e.g., organicEL elements) or plasma display panels (hereinafter, “PDP”) are used havebeen developed. Such display panels include a light-emitting layerdisposed between an anode electrode and a cathode electrode. Thelight-emitting layer emits light when a voltage is applied between theelectrodes. The light emitted from the light-emitting layer isrecognized as characters or images when viewed through a transparentmedium or a transparent panel that has a refractive index more than one.

The light from the light-emitting layer is radiated, or in other wordsemitted in almost all directions (emitted at all angles). Therefore,light that has an angle of incidence not less than a critical angle withrespect to an interface between a transparent medium and an externalmedium undergo total reflection at the interface and confined in thedisplay panel. For example, it is known that light emitted out of thedisplay panel, which uses the organic EL elements, is only about 20 to30 percent of total light emitted from the organic EL elements.

One approach to increase the emitted light, i.e., to increase efficiencyof the light or increase light extraction efficiency, inclined surfaceportions may be provided inside the panel. Such inclined surfaceportions allow light propagating at not smaller than the critical angleto undergo reflection or refraction, and therefore, the light isdirected at an angle smaller than the critical angle. Particularly,Japanese Patent Application Laid-open Publication No. 10-189251discloses a top-emission display panel in which light is emitted from aside of a transparent panel covering a light-emitting layer formed on abase substrate. At a periphery of the light-emitting layer, a wedgedreflecting member is disposed, and thereby a reflecting structure isformed. Moreover, Japanese Patent Application Laid-open Publication No.2001-332388 discloses a bottom-emission display panel in which light isemitted from a side of a base substrate on which a light-emitting layeris formed. In this bottom-emission display panel, a structure in whichinclined surface portions are formed on an anode and a cathode whichsandwich the light-emitting layer.

However, in such structures in which angle of light emitted from thelight-emitting layer is changed by the inclined surface portions andthereby total reflection in the interface is inhibited, the displaypanel is susceptible to become thick. This may impose restriction onmaking of a very thin flat panel display. To improve the lightextraction efficiency from the display panel, it is necessary to changeby the inclined surface portions the angle of light incident on theinterface between the transparent panel and the external medium at notless than the critical angle. For this, it is necessary to design astructure such that this light or a major part of this light is incidentwithout fail on the inclined surface. In other words, to reduce lightincident on the interface between the light-emitting layer and thetransparent panel at the critical angle, it is necessary to form highinclined surface portions. This results in thickening of the transparentpanel. Therefore, to improve the light extraction efficiency, a thicktransparent panel has to be used. This means that the thickness of thedisplay panel is not reduced. On the other hand, if the transparentpanel is made thin, the display panel can be made thin. However, theamount of light incident on the interface at not less than the criticalangle increases. This results in deteriorating the light extractionefficiency.

Furthermore, the structure to change the angle of the light does notallow all the light emitted from the light-emitting element to be outputfrom the display panel to the external medium.

In a display panel in which a protective layer is deposited on a displaylayer that includes the light-emitting element, not all the output lightfrom the light-emitting element is output to the external medium. One ofthe causes of this is that there is an interface also between thelight-emitting element and the protective layer and if the refractiveindex of the protective layer is lower than the refractive index of thelight-emitting element, there is a critical angle when the light isinput from the light-emitting element to the protective layer. Forexample, in a general structure of a display panel in which the organicEL elements are sandwiched between the two electrodes as light-emittingelements, a glass substrate is disposed as a protective layer on thedisplay layer formed on a substrate. The refractive index of glass is1.5 while the refractive index of the light-emitting particle is 1.7.The light emitted from the light-emitting element and passed through anelectrode contains a component that undergoes total reflection at theinterface. Therefore, out of the light emitted from the light-emittingelement, the light having an angle of incidence on the glass substratenot less than the critical angle is reflected at the boundary of theglass substrate and this light cannot be extracted.

If the refractive index of the transparent panel on the display layer asa protective layer or an output layer is made greater than therefractive index of the light-emitting element, the light that is inputfrom the display layer to the transparent panel does not contain acomponent of total reflection. Therefore, the light emitted from thelight-emitting element can be input to the transparent panel. However,if the refractive index of the transparent panel is increased, thedifference between the refractive index of the transparent panel andthat of air (i.e., an external medium) becomes greater and the criticalangle of the light with respect to an interface between the transparentpanel and the external medium becomes smaller. Due to this, even if thetransparent panel having a refractive index greater than that of thelight-emitting element is used, the amount of light that is extracted inthe external medium does not increase. Therefore, the light extractionefficiency cannot be improved.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the problemsin the conventional technology. The present invention enables to providea self-emitting element, a display panel, a display apparatus, whichimprove light extraction efficiency and thereby more light emitted fromthe light-emitting element is output to an external medium, and a methodof manufacturing the self-emitting element.

The present invention allows light emitted from a light-emitting layerto be incident on an interface at greater than a critical angle, thatis, allows the light to undergo total reflection. Whereas, the lightthat has undergone total reflection is reflected on a reflective layerformed on an opposite side of the interface, and thereby multiplereflection is caused to occur. The light with the multiple reflection isguided to an angle changer disposed at a periphery of the light-emittinglayer and the angle (i.e., propagating direction) of light is changed.

A self-emitting element according to the present invention includes alight-emitting layer that is disposed between electrodes and that emitslight upon applying a voltage between the electrodes; a protective layerthat covers an emitting side of the light-emitting layer, forms aninterface between the protective layer and an external medium, and has athickness that allows the light emitted from the light-emitting layer toundergo total reflection at least once at the interface in an area ofthe corresponding light-emitting layer; a reflective layer that coversan opposite side of the protective layer with respect to thelight-emitting layer; and an angle changer that is disposed at aperiphery of the light-emitting layer, and changes a direction of thelight propagating in the protective layer so that the light is incidenton the interface at less than a critical angle.

In the self-emitting element according to the present invention, a thinprotective layer having a thickness that allows the light emitted fromthe light-emitting layer to undergo total reflection at least once, isemployed. This thin protective layer enables to make the overallself-emitting element. Similar is a case in a display panel that isequipped with a plurality of light-emitting layers. Thus, by utilizingthe total reflection at the interface actively, it is possible toimprove the light extraction efficiency and to make the display panelthin. On the other hand, the light emitted at not less than the criticalangle with respect to the interface of the protective layer is subjectedto multiple reflection by the interface and the reflective layer on theopposite side of the interface. Due to the multiple reflection, thelight reaches the angle changer disposed at a periphery of thelight-emitting layer. Here, the angle (direction) of the light ischanged and the light is output to the external medium. Since it isadvisable to have a thin protective layer, an inclined structure thatforms the angle changer can be made thinner than a bank or a protrusionthat is mentioned below. With such a structure, the light that hasreached at a periphery of the light-emitting layer due to the multiplereflection is extracted with the angle (direction) changed by the anglechanger without fail. Therefore, the light extraction efficiency isincreased. Moreover, a high inclined surface is not required as theangle changer. This reduces time and labor required for making theinclined structure, thereby enabling to provide the display panel at alow cost.

Thus, it is possible to provide the self-emitting element and thedisplay panel that can satisfy the two contradictory requirements, i.e.,the improvement in light extraction efficiency and the reduced thicknessof the self-emitting element or the display panel. Particularly, in aself-emitting element or a display panel in which an organicelectro-luminescent light-emitting layer is employed, the lightextraction efficiency is said to be 20 to 30 percent at the most.Therefore, the present invention is very effective. Thus, a very thindisplay apparatus that includes the display panel according to thepresent invention and the drive unit that displays an image by drivingthe light-emitting layer of the display panel can be provided and it ispossible to display a bright image.

One aspect of the angle changer that changes the angle of lightpropagated by the protective layer and makes it smaller than thecritical angle, is a reflective surface that is inclined to have a wideemitting side. Further, the angle changer may also be a refractivesurface that is inclined to have a narrow emitting side. In any of thecases, the inclined structure is necessary.

If the inclined surface that encloses the surrounding area of thelight-emitting layer is high and has a constant slope, the higher theheight of the inclined surface is, the longer the distance between thelight-emitting layers is. This results in bigger size of the displaypanel that displays an image with high resolution. Consequently, adisplay apparatus in which this display panel is used becomes bigger. Inother words, if the inclined surface (the angle changer) is low, thedistance between the light-emitting layers is short. This allows toreduce the size of a display panel and a display apparatus that displaysan image with high resolution, and to make them compact.

Moreover, in a display panel in which a flat surface area that includesthe light-emitting layer and the inclined surface corresponds to onepixel, the inclined surface (the angle changer) is included in an areaof the pixel with respect to an area (light-emitting surface of thelight-emitting layer) of light emission per pixel. Since the area of theangle changer is reduced by the present invention, the area of the pixelcan be reduced without reducing the amount of light emitted from thepixel. This means that brightness of the display panel is improved.Thus, the present invention leads to an advantage of having highbrightness by reducing a pixel size and enables to display very brightimages or characters.

In the self-emitting element and the display panel, one of the twoelectrodes that sandwich the light-emitting layer, in other words anelectrode that is on the opposite side of the emitting side, can be madeto be the reflective layer. By doing so, a special reflective layer isunnecessary, and a thinner self-emitting element and a display panel canbe provided.

In the self-emitting element and the display panel according to thepresent invention, it is advisable to form a bank that projects on theemitting side to separate the light-emitting layers from each other andto make an inner surface of the bank as the angle changer. Theself-emitting element and the display panel according to the presentinvention may be constructed so that a sheet or a panel that includesthe angle changer is stuck as the protective layer. However, in thisconstruction, light (cross talk) tends to leak through a gap between thelight-emitting layer and the protective layer. Moreover, bubbles enterthe gap easily during sticking the protective layer. The bubbles maycause scattering of light thereby hindering the light efficiency.Further, it may be difficult to manufacture it without cross talk orentry of bubbles.

For this reason, the protective layer is formed in a region surroundedwith the bank by making the inner surface of the bank that is formed ata periphery of the light-emitting layer as the angle changer. Thisallows the protective layer to be disposed very close to thelight-emitting layer. In other words, it is possible to reduce thedistance between the protective layer and the light-emitting layer. As aresult, the cross talk is reduced. Moreover, forming the protectivelayer in the region surrounded with the bank may be achieved by castcoating of material of the protective layer. This cast coating enablesto avoid the entry of bubbles during the formation of the protectivelayer. With this structure, thickness of the protective layer isabsorbed in the thickness of the bank. In other words, the thickness ofthe protective layer does not affect the thickness of the self-emittingelement or the display panel, and a thinner self-emitting element anddisplay panel can be provided.

Therefore, it is possible to manufacture the self-emitting element withhigh light efficiency by a method of manufacturing a self-emittingelement, wherein the self-emitting element includes a light-emittinglayer that is disposed between electrodes and that emits light uponapplying a voltage between the electrodes; a protective layer thatcovers an emitting side of the light-emitting layer, forms an interfacebetween the protective layer and an external medium, and has a thicknessthat allows the light emitted from the light-emitting layer to undergototal reflection at least once at the interface in an area of thecorresponding light-emitting layer; a reflective layer that covers anopposite side of the protective layer with respect to the light-emittinglayer; and an angle changer that is disposed at a periphery of thelight-emitting layer, and changes direction of the light propagating inthe protective layer so that the light is incident on the interface atless than a critical angle, and the method includes forming a bank, asthe angle changer, that projects on the emitting side to separate thelight-emitting layer from other light-emitting layer; and forming theprotective layer in an area that is enclosed with the bank.

Furthermore, a protrusion made of an insulating material is formed so asto project toward the emitting side from the bank, and an inner surfaceof the protrusion may be made as the angle changer. The protective layeris formed in a region surrounded with the protrusion. However, comparedto the structure in which the inner surface of the bank is made to bethe angle changer, there is a possibility of occurrence of cross talk inthe bank. Therefore, the structure in which the inner surface of thebank is made to be the angle changer is the most desirable. On the otherhand, in the structure in which the inner surface of the bank is made tobe the angle changer, when a film like that of aluminum etc. is appliedon the inner side of the bank to make the angle changer, the electrodesthat are disposed so as to sandwich the light-emitting layer may getshort circuited by the angle changer. Therefore, it is necessary todeposit an insulating film to avoid the short circuit. Thus, thestructure in which a protrusion is formed on the bank and the innersurface of the protrusion is made to be the angle changer is suitable inview of the ease of manufacturing the display panel.

A self-emitting element in which an insulating protrusion is formed onthe bank and this protrusion is made to be the angle changer, can bemanufactured by a method that includes forming a protrusion as the anglechanger with an insulating material to separate the light-emitting layerfrom other light-emitting layer so that the protrusion is protruded froma bank that projects on the light-emitting side; and forming theprotective layer in an area that is enclosed with the protrusion.

The present invention can be applied to a self-emitting element or adisplay panel. Thus, the present invention is applicable to a displayobject or a display panel in which the PDP, a light-emitting diode(hereinafter, “LED”), an inorganic EL element, an organic EL element, ora field emission element is used. Particularly, a display object (or aself-emitting element) or a display panel in which organic EL elementsincluding an organic EL light-emitting layer as the light-emitting layeris used has a very low light extraction efficiency. Therefore, thepresent invention is very useful.

Moreover, in the present invention, refractive index of a transparentoutput layer that includes the angle changer, which changes thedirection of the light emitted from the light-emitting element into thedirection of emission, is the same or greater than the refractive indexof the light-emitting element. This is to achieve input of the light inthe output layer without any leakage of light. Also, the light incidenton the output layer is directed at an angle smaller than the criticalangle of the interface between the output layer and the external medium.This is to enable output of the output light efficiently to the externalmedium even if the output layer has a high refractive index. In otherwords, the display object (self-emitting element) according to thepresent invention includes a display layer that includes thelight-emitting element and the transparent output layer that includesthe angle changer. The angle changer is disposed in the direction ofemission of the display layer, and directs the light emitted from thelight-emitting element in the direction of emission. The refractiveindex of the output layer is the same as or greater than the refractiveindex of the light-emitting element.

By making the refractive index of the output layer same or greater thanthat of the light-emitting layer, the light directed from thelight-emitting element or the display layer to the output layer has nocomponent that undergoes total reflection. All the light emitted fromthe front or the top of the light-emitting element excluding lightemitted from the bottom of the light-emitting element is extracted inthe output layer. By providing in the output layer the angle changerthat changes the direction of the light by reflection or refraction, thedirection of the light having a bigger angle of incidence with respectto the interface between the output layer and the external medium ischanged so that the angle of incidence with respect to the interface issmaller. Even if the critical angle in the boundary with the externalmedium becomes smaller due to increase in the refractive index, it canbe input to the external medium by making the angle of incidence smallerthan the critical angle. Thus, the light extraction efficiency can bemaintained or improved. Therefore, the self-emitting element (thedisplay object) according to the present invention enables to reduceloss when the light is incident on the output layer from thelight-emitting element or the display layer. As a result, the efficiencyof light emitted from the light-emitting element can be improved.

A micro lens or a micro prism that changes the direction of light byrefraction or a micro mirror that changes the optical path by reflectingthe light can be employed as the angle changer that changes the opticalpath of the light in the output layer. The micro lens can also be usedas the output layer.

Even if an output layer having a high refractive index is prepared, ifan intermediate layer having a refractive index smaller than that of thelight-emitting element is disposed between the light-emitting elementand the output layer, the light incident on the output layer is reducedby a component that undergoes total reflection at the interface of theintermediate layer. On the other hand, even if an intermediate layerhaving a refractive index greater than that of the output layer isdisposed between the light-emitting element and the output layer, thelight emitted from the light-emitting layer is incident on the outputlayer at less than the critical angle through the intermediate layer.Therefore, there is no loss of light in the intermediate layer. For thisreason, it is necessary that the refractive index of the intermediatelayer is greater than that of the light-emitting layer. If thelight-emitting element is an organic EL that emits light when a voltageis applied, it is necessary to deposit a transparent layer, which is anelectrode for applying voltage to the light-emitting element, on thedisplay layer. This transparent electrode layer becomes an intermediatelayer between the light-emitting element and the output layer.Therefore, by making the refractive index of this transparent electrodelayer greater than that of the light-emitting element, the loss of theoutput light in the transparent electrode layer can be minimized.

On the other hand, when the difference between the refractive indices ofthe output layer and the transparent electrode layer is high, there is areflection of light at an interface of the output layer and thetransparent electrode layer due to difference in the refractive indexand the efficiency of light is reduced. Moreover, when an external lightis incident, due to reflection of the external light at the interface,the contrast is reduced. Therefore, it is desirable to improve theefficiency of light by providing an anti-reflective layer in theinterface between the transparent electrode layer and the output layerand also minimize the reflection of the external light.

Light output from the output layer functions effectively when passingthrough air (refractive index of air is approximately one) and reachingeyes of a user. A transparent sealing layer may be provided in thedirection of emission of the output layer across a space from the outputlayer. In this case, even if the space is filled with an inert gashaving a refractive index of approximately one and the sealing layer isformed by a material having a refractive index of approximately 1.5, allthe light input from the output layer to an inert gas layer can be inputto the sealing layer. Further, since the refractive indices of the inertgas layer and an air layer on the user side are almost the same, all thelight input to the sealing layer can be extracted in air.

In the present invention, in addition to adoption of the thin protectivelayer, the refractive index of the output layer is made same or greaterthan that of the light-emitting element. By doing so, the lightextraction efficiency can be improved further.

Thus, the self-emitting element (display object) according to thepresent invention can improve further the efficiency of the lightemitted from the light-emitting element, thereby enabling to outputlight having high brightness. Therefore, a display panel in which aplurality of self-emitting elements (display objects) are arranged twodimensionally in a matrix form can provide sharp images having highbrightness. Moreover, a display apparatus that includes the displaypanel according to the present invention and the drive unit thatdisplays images by driving the light-emitting element of the displaypanel makes it possible to display even brighter image at low electricpower.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a display apparatus (a portable telephone) inwhich a display panel according to the present invention is installed;

FIG. 2 is a top view of the display panel according to the presentinvention;

FIG. 3 is a cross sectional view of the display panel in FIG. 2;

FIG. 4 is a diagram indicating relationship between pixel area andlight-emitting area in the display panel in FIG. 2;

FIG. 5A to FIG. 5D are diagrams indicating process according to a methodof manufacturing the display panel in FIG. 2;

FIG. 6 is a cross sectional view of another display panel;

FIG. 7A to FIG. 7D are diagrams indicating process according to a methodof manufacturing the display panel in FIG. 6;

FIG. 8 is a cross sectional view of still another display panel;

FIG. 9 is a cross sectional view of still another display panel;

FIG. 10 is a cross sectional view of the display panel according to thepresent invention;

FIG. 11 is a graph indicating a relationship between refractive indexand incidence rate of an output layer of the display panel according tothe present invention;

FIG. 12 is a cross sectional view of a substrate on which a displaylayer is formed in the method of manufacturing the display panelaccording to the present invention;

FIG. 13 is a cross sectional view of a sheet on which a micro mirror isformed in the method of manufacturing the display panel according to thepresent invention;

FIG. 14 is a cross sectional view of the substrate on which the sheet isstuck in the method of manufacturing the display panel according to thepresent invention;

FIG. 15 is a cross sectional view of still another display panel;

FIG. 16 is a cross sectional view of still another display panel;

FIG. 17 is a cross sectional view of still another display panel;

FIG. 18 is a cross sectional view of still another display panel;

FIG. 19 is a cross sectional view of still another display panel; and

FIG. 20 is a cross sectional view of still another display panel;

FIG. 21 is a cross sectional view of still another display panel;

FIG. 22 is a cross sectional view of still another display panel.

DETAILED DESCRIPTION

The present invention is explained in detail below with reference to theaccompanying drawings. FIG. 1 is a diagram of a portable telephone as adisplay apparatus in which a display panel according to the presentinvention is installed. A portable telephone 1 in this embodimentincludes a display panel 10 a on which data is displayed and a driveunit 9. The display panel 10 a includes organic EL elements which isself-emitting elements. The drive unit 9 that includes a micro computercauses the organic EL element to emit light L and a user 90 views datathat includes characters and images.

FIG. 2 is a top view of the display panel in an enlarged form. FIG. 3 isa cross sectional view of a striped area III. The display panel 10 a inthis embodiment is formed by disposing a plurality of pixels from theorganic EL elements in a matrix form and can be driven by an activematrix or by a passive matrix. The display panel 10 a includes aplurality of display objects 19 or a self-emitting element 19 that isdisposed in a two dimensional direction in an array form or a matrixform. The display object 19 or a self-emitting element 19 forms onepixel and includes a separate light-emitting layer 14 and a reflectivesurface 13 a at a periphery of the light-emitting layer 14. Therefore, atwo dimensional image can be displayed by driving each of the displayobjects 19. Each of the display objects 19 is disposed betweenelectrodes and includes the light-emitting layer 14 and a protectivelayer 18. The light-emitting layer 14 emits light when a voltage isapplied between the electrodes. The protective layer 18 covers anemitting side of the light-emitting layer 14 and forms an interface 18 awith an external medium. The reflective surface 13 a that is an anglechanger provided at a periphery of the light-emitting layer, changes thedirection of light L2 that is propagated in the protective layer 18 toan angle less than a critical angle of the interface 18 a of theprotective layer. Thus, the efficiency of light and light extractionefficiency are improved by changing the angle.

The display panel 10 a further includes a substrate 11 that is a supportof a panel like a glass substrate. A cathode layer 12 (also called as anelectrode layer 12 or a reflective layer 12) is deposited on an uppersurface or a surface 11 a of the substrate 11. The cathode layer 12includes a signal line, a driving element, and a film of multiple layersof a dielectric substance or a metal like aluminum. Thus, in the displaypanel 10 a in this embodiment, the cathode layer 12 is a reflectivelayer and has a function to reflect a part of light emitted from thelight-emitting layer 14 toward the protective layer 18 and to reflectlight that undergoes total reflection at the interface 18 a toward theprotective layer 18. Further, a bank 13 of a predetermined height madeof polyimide is deposited in a predetermined pattern on the uppersurface 11 a of the substrate 11. An area 21 surrounded with banks 13from four sides is formed. The area 21 is an area of forming the organicEL light-emitting layer 14 and is a light-emitting area. In thisembodiment, the light-emitting area 21 that is rectangular in shapehaving a size of 60 μm×90 μm is formed. An area 22 (hereinafter “pixelarea”), which includes inclined side surfaces (inclined surfaces) of thelight-emitting area 21 and the bank 13, corresponds to a pixel in thedisplay panel 10 a and for example is a pixel area of a size 80 μm×240μm.

The light-emitting layer 14 is deposited on the area 21 using ink-jettechnology. The bank 13 is a layer that can be used for alignment duringdepositing the light-emitting layer 14 and separates the light-emittinglayer 14. The light-emitting layer 14 may be a separate layer thatincludes an organic EL element or it can be a layer to which a holetransport layer or an electron transport layer is added. A transparentelectrode (an anode layer) 15 made of indium tin oxide (hereinafter,“ITO”) is formed on the light-emitting layer 14. When a voltage isapplied to the cathode layer 12 and the anode layer 15, thelight-emitting layer 14 disposed between these electrodes emits light.The protective layer 18 is deposited on the anode layer 15(light-emitting side) and the emitting side of the light-emitting layer14 is covered by the protective layer 18. The interface 18 a is formedbetween the protective layer 18 and the external medium. In this case,the electrode layer 12 is a cathode and the electrode layer 15 is ananode. However, if the electrode layer 12 is a reflecting electrode andif the electrode layer 15 is an optically transparent layer, there is noneed to restrict to the above mentioned combination.

The protective layer 18 is a thin and transparent layer. The protectivelayer 18 has a thickness that allows the light emitted from thelight-emitting layer 14 at an angle greater than the critical angle withrespect to the interface 18 a to undergo total reflection at theinterface 18 a inside the area of the light-emitting layer 14. In aconventional display panel that changes an angle at the inclinedsurface, a protective layer has a thickness that does not allow thelight emitted from the light-emitting layer at an angle such that itundergoes total reflection, to reach an interface between the protectivelayer and the external medium. Whereas, in the display panel 10 a inthis embodiment, the protective layer 18, which has a thickness thatallows the light emitted at not less than the critical angle to bereflected at the interface 18 a, is deposited thinly. Therefore, out ofthe light L emitted from the light-emitting layer 14, light L1 emittedat an angle smaller than the critical angle with respect to theinterface 18 a is output from the interface 18 a to the external medium.Out of the light L, the light L2 emitted at an angle not smaller thanthe critical angle with respect to the interface 18 a undergoes totalreflection and returns toward the light-emitting layer 14 and isreflected once again at the electrode layer 12. The light L2 ispropagated inside the protective layer 18. For example, if theprotective layer 18 has a refractive index of 1.5, the light L2 incidenton the interface 18 a at not less than about 42 degrees undergoes totalreflection at the interface 18 a and is propagated inside the protectivelayer 18.

The light L2 propagated in the protective layer 18 reaches the inclinedsurface of the bank 13 at a periphery of the light-emitting area 21. Theangle of the light L2 is changed to less than the critical angle. Due tothis, the light L2 passes through the interface 18 a and is output tothe external medium. In other words, a side surface 13 a (hereinafter,“reflective surface 13 a”) of the bank 13 is inclined so that it becomeswider in the direction of the light-emitting side. A reflecting film 24of aluminum etc. is deposited on the side surface 13 a. Due to this,direction (angle) of the light L2 subjected to repetitive reflectionbetween the interface 18 a and the reflective layer 12 is changed at thereflective surface 13 a. Therefore, the light L2 reflected from thereflective surface 13 a is incident on the interface 18 a at smallerthan the critical angle and is output. For this reason, in the displaypanel 10 a in this embodiment, the light L2 that undergoes totalreflection at the interface 18 a due to the use of the thin protectivelayer exists. However, since the light L2 reaches the reflective surface13 a due to multiple reflection and is output to the external mediumafter having changed the direction, there is hardly any light that isconfined due to total reflection. This enables to provide a displaypanel that has efficiency of light and light extraction efficiencysimilar to or better than those of the conventional display panelequipped with a thick protective layer and a high reflective surface.

In the conventional display panel, the protective layer is formed sothat a ray of light emitted from the light-emitting layer to theinterface between the protective layer and the external medium is notreflected totally at the interface. Therefore, it is necessary to makethe protective layer thicker. The thickness varies depending on thelight-emitting area or the pixel area but the thickness of theprotective layer becomes same or greater than the pixel size. Forexample, if the light-emitting area or the pixel area is 50 μm×50 μm,the required thickness of the protective layer 18 is 70 μm. For thedisplay panel 10 a in this embodiment, since it is assumed that thelight is reflected totally at the interface 18 a of the protective layer18 and the external medium, the protective layer 18 can be made thinnerto the limit of performing the function of protection. Therefore, forthe light-emitting area or the pixel area measuring 50 μm×50 μm, it ispossible to manufacture a display panel having the very thin protectivelayer 18 of a few micro meters. This enables to reduce the thickness ofthe protective layer to one tenth, thereby realizing a very thin displaypanel.

Moreover, the reflective surface 13 a at a periphery of thelight-emitting layer 14 is for changing the direction of light subjectedto multiple reflection between the protective layer 18 and thereflective layer 12. The reflective surface 13 does not change thedirection of the light such that the light emitted from light-emittinglayer 14 by reflecting once is not reflected totally at the interface 18a. Therefore, the height at which the light emitted from thelight-emitting layer 14 at an angle such that it undergoes totalreflection, is not necessary as in the conventional display panel. Thisenables to reduce area per pixel. FIG. 4 is an illustration indicating arelationship between the area occupied by the reflective surface 13 aand the light-emitting area 21 in the pixel area 22. If the angle of thereflective surface of the display panel 10 a is made same as that of thereflective surface of the conventional display panel, the height can bereduced. This enables narrowing of the area that is required to form thereflective surface 13 a. Therefore, it is possible to form thelight-emitting area 21 that emits same amount of light as in theconventional art with respect to the pixel area 22 and the brightnessper pixel increases. Moreover, since the light propagated through theprotective layer 18 is output from the surrounding of the pixel, thedisplay object 19 has an amount of light in the surrounding equivalentto the light at the center or more. This enables a display having a highcontrast and clear outline.

To put it the other way round, in the conventional art, since the pitchbetween two pixels is limited, the height of the reflective surface islimited. Therefore, the amount of light output from the interface afterhaving changed the angle by the reflective surface is limited. Whereas,in the present invention, since the protective layer can be made thinand the height of the reflective surface (inclined surface) 24 can alsobe reduced, it is possible to make an inclined surface of the sameheight as that of the protective layer 18 or the height greater thanthat of the protective layer 18. This prevents escaping of light that ispropagated in the protective layer 18 and enables to output the lightfrom the interface 18 a after having changed its direction. Due to this,it is possible to provide the thin display panel 10 a having high lightextraction efficiency. However, when the protective layer 18 is madethin, the number of reflections in multiple reflection increases. Thereflection at the interface 18 a is total reflection and there is notmuch loss. However, the light is absorbed at metal surfaces likealuminum of the electrode 12 and also during the propagation in theprotective layer 18. Thus, the making of the protective layer 18 thinresults in loss due to absorption and this may affect the lightextraction efficiency.

In any of the cases, in the display panel 10 a in this embodiment, it ispossible to make the protective layer thin without reducing the lightextraction efficiency or with improvement in the light extractionefficiency. This structure enables to achieve two opposite requirementsviz. improvement in the light extraction efficiency and making of a thindisplay panel. Thus, a bright and thin display panel can be manufacturedor provided. Further, a low reflective surface can be used and not onlythe improvement in light extraction efficiency but also the improvementin the brightness per pixel can be easily achieved together. Thus, it ispossible to provide a display panel that can display a very brightimage. Further, in the display panel 10 a, since the electrode 12disposed on a side opposite to the emitting side of the light-emittinglayer 12 is also used as a reflective surface, this is the most suitablearrangement for manufacturing a thin display panel. Moreover, there isno need to consume energy more than that is required. This saves thepower consumption of a display apparatus 1 and enables to improve thereliability of the organic EL element.

FIG. 5A to FIG. 5D are diagrams indicating process in a method ofmanufacturing of the display panel 10 a. As is shown in FIG. 5A, asubstrate 11, like a glass substrate is arranged and the electrode layer(cathode layer) 12 made of aluminum is deposited on the surface 11 a.The bank 13 formed with polyimide is formed on the electrode layer 12.The reflecting film 24 which is an aluminum film or a film of multiplelayers of a dielectric substance is formed on the side surface (inclinedsurface) 13 a of the bank 13. While forming the conductive reflectingfilm of aluminum etc., it is necessary to form an insulating film toavoid short circuit between the electrode 12 and the electrode 15.

Further, as is shown in FIG. 5C, the light-emitting layer 14 isdeposited on area that is surrounded with the bank 13 by impact or bydropping ink solution including an organic EL material by ink-jetmethod. Further, the electrode layer (anode layer) 15 is deposited onthe light-emitting layer. Then, as is shown in FIG. 5D, the thinprotective layer 18 is deposited on the emitting side of the substrate11 formed by the electrode layer 12, the bank 13, the light-emittinglayer 14, and the electrode layer 15. Thus, the display panel 10 a ismanufactured.

FIG. 6 is a cross sectional view of another display panel. In thedisplay panel 10 a mentioned above, the reflective surface 13 a isformed on the bank 13 that surrounds the light-emitting layer 14 and thereflective surface 13 a is disposed on side of the light-emitting layer14. In other words, there is no gap between the reflective surface 13 aand the light-emitting layer 14 in a vertical direction or in adirection of emission. Thus, this structure prevents the leakage oflight (cross talk) emitted from the light-emitting layer 14 and themaximum efficiency of light is achieved. Further, in this structure, theprotective layer 18 is deposited to a small extent on the bank 13 but itis desirable that the protective layer 18 is on the anode layer 15. Itis possible to form a structure in which the anode layer 15 is coveredby the protective layer 18 having a thickness less than the height ofthe bank 13. This enables to have a very thin display panel. However, inthis display panel 10 a, to prevent short circuit between theelectrodes, there is a need to form an insulating layer while formingthe reflective layer 13 a which may complicate the manufacturing of thedisplay panel to a small extent.

Whereas, in a display panel 10 b that is shown in FIG. 6, the reflectingfilm 24 is not formed on the inclined surface 13 a of the bank 13 butthe anode 15 is formed on the bank 13 and the light-emitting layer 14.Further, the protrusion 25 made of an insulating material is formed onthe anode 15 in a position piled up on the bank 13. The reflecting film24 is formed on an inclined surface 25 a of the protrusion 25. The areasurrounded with the protrusion 25 is covered by the protective layer 18.With this display panel 10 b, there is no concern of short circuit ofelectrodes by the reflecting film 24 and there is no need to form aninsulation film to prevent short circuit between the electrodes. Thisfacilitates manufacturing of the display panel 10 b. However, sincethere is a gap between the light-emitting layer 14 and the reflectivesurface 25 a in the direction of emission, there may be a leakage oflight from a part of the bank 13. The height of the protrusion 25 in thedisplay panel 10 b can be kept about the same as that of the bank or maybe varied. To make the control panel thin, it is desirable that theheight of the protrusion 25 is low. On the other hand, when theprotective layer 18 is required to be thick enough to perform thefunction of protection, the thickness of the protective layer 18 can becontrolled freely by adjusting the height of the protrusion 25.

The display panel 10 b can be manufactured by the following method. Tostart with, similarly as in the case of the display panel 10 a, theelectrode 12, the bank 13, and the light-emitting layer 14 are formed onthe surface 11 a of the substrate 11 shown in FIG. 7A. As is shown inFIG. 7C, the electrode layer 15 is formed on the light-emitting layer 14and the bank 13. Further, a protrusion 25 made of an insulating materialis formed on the bank 13 (i.e., piling up on the bank 13). Thereflecting film 24 is formed on the inclined surface 25 a of theprotrusion 25. Then, as is shown in FIG. 7D, the protective layer 18 isformed by casting a material that forms the protective layer in the areasurrounded with the protrusion 25. Thus, the display panel 10 b ismanufactured.

FIG. 8 is a cross sectional view of still another display panel 10 c.The display panel 10 c includes the bank 13, a transparent layer 30, anda refractive surface 31. The transparent layer is piled up on the bank13 and has a refractive index different than that of the protectivelayer 18. The refractive surface 31 is formed as the angle changer sothat the direction of emission becomes narrow. Thus, the angle changercan also realize the refractive surface apart from the reflectivesurface.

FIG. 9 is a cross sectional view of still another display panel 10 d.The display panel 10 d includes a protective sheet 18 that has aprotrusion 28. The reflecting film 24 is formed on a wall surface(inclined surface) of the protrusion 28. The protective sheet 18 isjoined to the emitting side of the substrate 11 where the bank 13 isformed, through an adhesive layer 26. In the case of this structure, ifthe distance between the apex of the mirror 24 and the interface 18 a ofthe protective layer 18 is long, there is a leakage of light from thegap in between. Therefore it is desirable to prevent the leakage oflight by shortening these distances (gaps) to the possible extent.

In all the embodiments mentioned above, the reflective surface isinclined such that it is becoming wider toward the light-emitting side.It is also possible to have the reflective surface inclined so that itis becoming narrow toward the light-emitting side. However, if thereflective surface is becoming wider toward the light-emitting side, thenumber of reflections between the interface 18 a and the reflectivelayer 12 can be decreased. Therefore, considering the absorption loss atthe reflective layer 12 or transmittance of the light-emitting layer 14etc., from point of view of minimizing the decline in the optical powerdue to the multiple reflection, it is desirable to have the reflectivesurface that becomes wider in the direction of emission.

FIG. 10 is an enlarged view of a part of cross section of a displaypanel 104 that is a self-emitting element included in another displaypanel 103. The display panel 103 includes a substrate 105, a displaylayer 110, and an output layer 130 that are deposited in order on thesubstrate 105. The display object 110 is separated by a bank 112 made ofpolyimide. The organic EL element disposed two dimensionally in a matrixform is taken as a light-emitting element 111 which is a light-emittinglayer. This light-emitting element 111 is sandwiched between anelectrode 120 and an electrode 121. Out of these two electrodes, theelectrode 120 on the side of the output layer 130 is a transparentelectrode of a material like ITO. Therefore, for each light-emittingelement 111, the substrate 105, the display layer 110, and the outputlayer 130 are deposited one above the other to form the display object104. The display panel 103 includes a plurality of the display objects104 disposed in the form of a matrix.

In the display panel 103 that includes the organic EL elements as thelight-emitting elements, the refractive index of the light-emittingelement is 1.7. If the transparent electrode layer 120 is made of ITO,the refractive index becomes approximately 2.0.

The whole of the output layer disposed on the light-emitting side of thetransparent electrode layer 120 is transparent. A sheet 131 is providedwith a reflecting plate 132 that reflects light 108 emitted from thelight-emitting element 111. The sheet 131 having the reflecting plate isstuck to the display layer 110 by a transparent adhesive layer 133.

The sheet 131 and the adhesive layer 133 in the present embodiment areformed by an aryl resin that includes a large number of multiple bondslike a double bond and a triple bond, and the refractive index is madeto be approximately 1.7. Therefore, in the display panel 103 in thisembodiment, the refractive index of the output layer is about the sameas that of the light-emitting element 111. Due to this, at a firstinterface 130 b between the display layer 110 and the output layer 130,out of the light emitted from the light-emitting element 111, there isno component in the output light 108 that is output in a direction D1(direction of emission or forward direction) of a user 109 and thatundergoes total reflection. Thus, the whole of the output light 108 fromthe light-emitting element 111 is transmitted to the output layer 130.

Actually, the refractive index of the transparent electrode layer 120 isabout 2.0 and when the refractive index of the output layer 130 is lessthan that of the transparent electrode layer 120, there is a totalreflection of the output light 108 at the first interface 130 b.However, the whole of the light 108 transmitted from the light-emittingelement 111 to the transparent electrode layer 120 is input to the firstinterface 130 b at an angle not greater than the critical angle.Therefore, even if the refractive index of the output layer 130 is lessthan the refractive index of the transparent electrode layer 120, theoutput light 108 is not reflected totally at the first interface 130 b.Due to this, if the refractive index of the transparent electrode layer120 is either same or greater than that of the light-emitting element111, by comparing the refractive indices of the light-emitting element111 and the output layer 130, the judgment of whether the output light108 from the light-emitting element 111 is transmitted to the outputlayer 130 or not can be made.

On the other hand, when the refractive index of the transparentelectrode layer 120 is less than the refractive index of thelight-emitting layer 111, a part of the output light 108 is reflectedtotally at the interface of the transparent electrode layer 120 and thelight-emitting element 111. Therefore, there is a decline in theefficiency of the output light 108.

In the display panel 103 of this embodiment, out of the output light 108input to the output layer 130, the light input to an interface 130 a atan angle smaller than the critical angle at the interface 130 a being aninterface with the external medium, passes through the interface 130 aand is output to the external medium. Whereas, out of the output light108 input to the output layer 130, the light having an angle ofincidence greater than the critical angle is reflected by the reflectingplate 132 on the sheet 131 so that the angle of incidence at theinterface 130 a becomes smaller. Therefore, the light that is input tothe output layer 130 is output directly or after being reflected at thereflecting plate 132 passes through the interface 130 a and is output tothe external medium 150.

FIG. 11 is a graph indicating a relationship between the proportion ofincidence (incidence rate) of the output light 108 on the output layer130 and the refractive index of the output layer 130 when the refractiveindex of the light-emitting element 111 is 1.7. If the refractive indexof the output layer 130 becomes greater than that of the light-emittinglayer 111, incidence ratio becomes one. This indicates that there is noloss due to total reflection. When a glass substrate that containssilica as a main component is used, since the refractive index isapproximately 1.5, the incidence ratio becomes 0.78. Therefore, byincreasing the refractive index of the output layer 130 than that of thelight-emitting element 111, the proportion of the output light input tothe output layer increases by approximately 30 percent. Due to this, ifthe output light 108 input to the output layer 130 is output to theexternal medium from the external layer 130, the efficiency of light canbe improved by about 30 percent according to the present invention.

Resins that contain multiple bond like double bond, triple bond etc. aredesirable as resins having a high refractive index of not less than 1.7and it is easy to achieve high refractive index with aryl resins.

FIG. 12 to FIG. 14 are cross sectional views that depict themanufacturing process of the display panel 103 in FIG. 10. As is shownin these diagrams, the substrate 105 on which the display layer 110having the organic EL element 111 deposited in the form of a matrix isprepared (FIG. 12). Apart from this, a protrusion 134 that istrapezoidal in shape is formed on the lower surface in the form of amatrix. The sheet 131 is prepared by providing the reflecting plates 132on inclined surfaces of the protrusion 134 (FIG. 13). Then, as is shownin FIG. 14, the sheet 131 is stuck by the adhesive layer 133 so that aside of the display layer 110 of the substrate is covered by the sheet131. Thus, the display panel 103 can be prepared manufactured. Thecompositions of or the materials of the sheet 131 and the adhesive layer133 may be the same or may be different. The composition or the materialis to be selected so that the refractive index of each layer is greaterthan the refractive index of the light-emitting element 111. One of theembodiments is an aryl resin. Besides this, high refractive index can beachieved by a resin that contains a large number of multiple bonds likedouble bond and triple bond.

As mentioned above, the output light 108 input to the output layer 130is output from the output layer to the external medium 150. To improvethe output efficiency of the light 108, the reflecting plate 132 is usedfor changing the direction of the light. The present invention isexplained with this embodiment of the display panel. However, refractionmay be used for changing the direction.

FIG. 15 is a cross sectional view of a different display panel 103 a.The display panel 103 a includes a display object 104 a disposed in amatrix form. The display object 104 a includes the output layer 130 thathas micro lens 136 on the inner side. The micro lens 136 changes thedirection of the output light 108 by the refractive surface.

The output layer 130 includes a lens sheet 135 and an adhesive layer133. The lens sheet 135 is processed so that the lower surface of themicro lens 136 forms a refractive surface. If the refractive index ofthe lens sheet 135 is the same as that of the adhesive layer 133, themicro lens 136 is not formed. Therefore, it is desirable that therefractive index of the adhesive layer 133 is greater than that of thelens sheet 135. Further, if the refractive index of the adhesive layer133 is greater than that of the light-emitting element 111, similar asin the display panel 103, the output light 108 of the light-emittingelement 111 is input to the output layer 130 without any leakage. Thedirection of the output light 108 is changed at the micro lens 136 andit can be output from the output layer 130 to the external medium 150.This enables to provide a display panel with even higher efficiency oflight.

FIG. 16 is a cross sectional view of another display panel 103 b. In thedisplay panel 103 b, the micro lens 136 is the output layer 130. In thedisplay panel 103 b, a display object 104 b is disposed in the form of amatrix. In the display object 104, a refractive surface being a microlens 136 is an interface 130 a with the external medium 150. In thedisplay panel 103 b, a surface of the micro lens 136 is the interface130 a. The angle of incidence of the output light 108 becomes smallerthan the critical angle by changing the inclination of the interface 130a. As a result, it is possible to output the output light to theexternal medium 150 with improved efficiency. Thus, the efficiency oflight is further improved.

These micro lenses 136 can be formed directly on the transparentelectrode layer 120 by ink-jet method. This enables to provide a thinand very bright display panel at low cost.

FIG. 17 is a cross sectional view of still another display panel 103 c.The display panel 103 includes a transparent sealing layer 140 on theemitting side of the micro lens 136. An inert gas 142 having arefractive index not less than one is filled inside the sealing layer140. The sealing layer 140 and the inter gas 142 prevent thedeterioration of the organic EL element 111 due to oxidation orabsorption of moisture. The display panel 103 c transmits the outputlight 108 to the user 109 through the external medium (air) having arefractive index of one and displays an image. For this reason, if therefractive index of the inert gas 142 inside the sealing layer 140 isless than one, the critical angle at the interface 130 a between theoutput layer 130 and the inert gas 142 becomes smaller than the criticalangle at the interface with air, thereby increasing the loss due to thetotal reflection. Whereas, if the refractive index of the inert gasbecomes greater than one, the refractive power of the micro lens 136decreases. Due to this, the efficiency of changing the direction of theoutput light 108 to D1 is declined. Moreover, if the refractive index ofthe sealing layer 140 is not less than that of the inert gas 142, thereis a loss due to the total reflection when the light is incident on thesealing layer 140. This imposes limitations on selection of material ofthe sealing layer 140. Therefore, it is desirable that the refractivepower of the inert gas is approximately one.

FIG. 18 is a cross sectional view of still another display panel 103 d.In this display panel 103 d a display object 104 d is disposed in amatrix form. In the display object 104 d, an anti-reflective layer 145is deposited on the transparent electrode layer 120 and the output layer130 is formed by the micro lens 136. When there is a big difference inthe refractive index of the output layer 130 and that of the transparentelectrode layer 120, even if the light is incident at the interface ofthe output layer 130 and the transparent electrode layer 120 at lessthan the critical angle, there is a reflection due to the difference inthe refractive index and the efficiency of light is declined. Moreover,external light 107 like sun light or illuminated light that is input tothe display panel 103 from the external medium 150 is reflected due tothe difference between the refractive index of the output layer 130 andthat of the transparent electrode layer 120. This declines the contrastof the output light 108 emitted from the light-emitting element 111. Toavoid this, the anti-reflective layer 145 is disposed between thedisplay layer 110 and the output layer 130, i.e. between the transparentelectrode layer 120 and the output layer 130. This improves theefficiency of light and prevents the reflection of the external light107. In the display panel 103, to prevent reflection due to thedifference in the refractive index of all layers, it is desirable todispose an anti-reflective at the interface of the light-emittingelement 111 and the transparent electrode layer 120. However, it is notpossible to dispose an insulating anti-reflective layer between thelight-emitting element 111 and the transparent electrode layer 120.Therefore, the anti-reflective 145 is disposed between the display layer110 and the output layer 130 and the reflection due to the difference inthe refractive indices is reduced as much as possible. This enables toprovide the display panel 103 d that can display a clear image with highcontrast and high light extraction efficiency.

In the display panels 103 to 103 d mentioned above, a material having ahigh refractive index is used for the output layer 130 to supply thelight 108 that is output from the light-emitting element 111 to theoutput layer 130 without leakage. The optical path is changed at theoutput layer 130 and the light is incident on the interface 130 a withthe external medium at an angle smaller than the critical angle thatbecomes smaller due to use of the material having a high refractiveindex. Thus, the efficiency of light is further improved. The directionof the output light 108 can be changed by reflection or refraction. In acase of changing the direction of the output light by refraction, it ispossible to use prism instead of restricting to lens.

According to the result of simulation in FIG. 11, with the presentinvention, compared to the case of not using the material having a highrefractive index, it is possible to improve the efficiency of lightfurther by about 30 percent. The present invention enables to provide adisplay panel that can display a clear image with high brightness at lowpower consumption.

FIG. 19 is a cross sectional view of still another display panel 203.The display panel 203 according to the present embodiment includes alarge number of pixels that are the organic EL elements and are disposedin a matrix form, and can be driven by an active matrix or by a passivematrix. The display panel 203 includes a plurality of display objects219 that are disposed in an array form or a matrix from in twodimensional direction. The display object 219 or a light element formsone pixel. The display object 219 includes a separate light-emittinglayer 214 and a reflective surface 213 a provided at a periphery of thelight-emitting layer 214.

Therefore, by driving each display object 219, a two dimensional imagecan be displayed. Each display object 219 is disposed between theelectrodes and includes a light-emitting layer 214 and a protectivelayer 218. The light-emitting layer 214 emits light when a voltage isapplied between the electrodes. The protective layer 218 is deposited onthe emitting side of the light-emitting layer 214 and forms an interface218 a with the external medium. A reflective surface 213 is an anglechanger provided at a periphery of the light-emitting layer 214. Thereflective surface 213 changes the direction of light L22 that ispropagated inside the protective layer 218 so that the angle is smallerthan the critical angle of the interface 218 a. This improves theefficiency of light and the light extraction efficiency of light.

The display panel 203 includes a substrate 211 like a glass substratethat is a supporting plate of a panel. A cathode layer 212 (also calledas an electrode layer 212 or a reflective layer 212) is deposited on anupper surface of the substrate 211. The cathode layer 212 includes asignal line, a driving element, and a film of multiple layers of adielectric substance or a metal like aluminum. Thus, in the displaypanel 203 in this embodiment, the cathode layer 212 is a reflectivelayer and has a function of reflecting a part of light emitted from thelight-emitting layer 214 toward the protective layer 218 and to reflectlight that undergoes total reflection at the interface 218 a toward theprotective layer 218.

Further, a bank 213 of a predetermined height made of polyimide isdeposited in a predetermined pattern on the upper surface 211 a of thesubstrate 211. An area 221 surrounded with banks 213 from four sides isformed. The area 221 is an area of forming of the organic ELlight-emitting layer 214 and is a light-emitting area.

The light-emitting layer 214 is deposited on the area 221 using theink-jet technology. The bank 213 is a layer that can be used foralignment during depositing the light-emitting layer 214 and separatesthe light-emitting layer 214. The light-emitting layer 214 may be aseparate layer that includes an organic EL element or it can be a layerto which a hole transport layer or an electron transport layer is added.A transparent electrode (an anode layer) 215 made of ITO is deposited onthe light-emitting layer 214. When a voltage is applied to the cathodelayer 212 and the anode layer 215, the light-emitting layer 214 disposedbetween these electrodes emits light. The protective layer 218 isdeposited on the anode layer 215 (light-emitting side) and the emittingside of the light-emitting layer 214 covered by the protective layer218. The protective layer 218 forms the interface 218 a with theexternal medium. In this case, the electrode layer 212 is a cathode andthe electrode layer 215 is an anode. However, if the electrode layer 212is a reflecting electrode and if the electrode layer 215 is an opticallytransparent layer, there is no need to restrict to the above mentionedcombination.

The protective layer 218 is a thin and transparent layer. The protectivelayer 218 has a thickness that allows the light emitted from thelight-emitting layer 214 at an angle greater than the critical anglewith respect to the interface 218 a to undergo total reflection at theinterface 18 a inside the area of the light-emitting layer 214. In aconventional display panel that changes an angle at the inclinedsurface, a protective layer, which has a thickness that does not allowthe light emitted from the light-emitting layer at an angle of totalreflection, to reach an interface between the protective layer and theexternal medium. Whereas, in the display panel 203 in this embodiment, afilm of the protective layer 218 that is deposited, has a thickness thatallows the light emitted at not less than the critical angle to reachthe interface 218 a and to be reflected from the interface 218 a.Therefore, out of the light L that is emitted from the light-emittinglayer 214, light L21 emitted at an angle smaller than the critical angleof the interface 218 a is output from the interface 218 a to theexternal medium and light L22 emitted at an angle not smaller than thecritical angle of the interface 218 a undergoes total reflection andreturns toward the light-emitting layer 214 and is reflected once againat the electrode layer 212. The light L22 emitted at not smaller thanthe critical angle, is propagated inside the protective layer 218.

The light L22 propagated in the protective layer 218 reaches theinclined surface of the bank 213 at a periphery of the light-emittingarea 221. The angle of the light L22 is changed and becomes less thanthe critical angle. Due to this, the light L22 passes through theinterface 218 a and is output to the external medium. In other words, aside surface 213 a of the bank 213 is inclined so that it becomes widein the direction of the light-emitting side. A reflecting film 224 ofaluminum etc. is deposited on the side surface 213 a. Due to this,direction (angle) of the light L22 that is subjected to repetitivereflection between the interface 218 a and the anode layer 212 that is areflective layer is changed at the reflective surface 214 a. Therefore,the light L22 reflected from the reflective surface 213 a is incident onthe interface 218 a at an angle smaller than the critical angle and isoutput. For this reason, in the display panel 203 in this embodiment,the light L22 that undergoes total reflection at the interface 218 a dueto the use of the thin protective layer, exists. However, since thelight L22 reaches the reflective surface 213 a due to multiplereflection and is output to the external medium after having changed thedirection, there is hardly any light that is confined due to the totalreflection. This enables to provide a display panel that has efficiencyof light and light extraction efficiency equivalent to or better thanthose of the conventional display panel equipped with a thick protectivelayer and a high reflective surface.

In the display panel 203 in this embodiment the refractive index of thelight-emitting layer 214 that is formed by the organic EL is 1.7. If theanode layer 215 that is a transparent electrode layer is made of ITO,the refractive index become approximately 2.0.

The protective layer 218 in the present embodiment is formed by an arylresin that includes a large number of multiple bonds like a double bondand a triple bond, and the refractive index is made to be approximately1.7. Therefore, in the display panel 203 in this embodiment, therefractive index of the protective layer 218 that is an output layer, isabout the same as that of the light-emitting layer 214. Here, thecathode layer 212, the light-emitting layer 214, and the anode layer 215form a display layer 210. At a first interface 230 b between the displaylayer 210 and the protective layer 218 that is an output layer, out ofthe light that is output from the light-emitting layer 214, there is nocomponent in the output light that is output in a direction D1 of a user209 that undergoes total reflection. Consequently, the whole of theoutput light from the light-emitting layer 214 is transmitted to theprotective layer 218.

Actually, the refractive index of the anode layer 215 that is atransparent electrode layer is about 2.0 and when the refractive indexof the protective layer 218 that is an output layer is less than that ofthe anode layer, there is a total reflection of the light at the firstinterface 230 b. However, the whole of the light that is transmittedfrom the light-emitting layer 214 to the anode layer 215 is input to thefirst interface 230 b at an angle not greater than the critical angle.Therefore, even if the refractive index of the protective layer 218 isless than the refractive index of the anode layer 215, the output lightis not reflected totally at the first interface 230 b. Consequently, ifthe refractive index of the anode layer 215 is either same or greaterthan that of the light-emitting layer 214, by comparing the refractiveindices of the light-emitting layer 214 and the output layer 230, thejudgment of whether the output light from the light-emitting layer 214is transmitted to the output layer 230 or not can be made.

On the other hand, when the refractive index of the anode layer 215 isless than the refractive index of the light-emitting layer 214, a partof the output light is reflected totally at the interface of the anodelayer 215 and the light-emitting layer 214. Therefore, there is adecline in the efficiency of the output light.

In the display panel 203 in this embodiment, out of the output lightinput to the output layer 230, the light input to a second interface 230a at an angle smaller than the critical angle at the interface 230 a isoutput to the external medium. Whereas, out of the output light input tothe output layer 230, the light having an angle of incidence greaterthan the critical angle is reflected by the reflective surface 213 a sothat the angle of incidence at the interface 230 a becomes smaller.Therefore, the light input to the output layer 230 is output directly orafter being reflected at the reflective surface 213 a passes through theinterface 230 a and is output to the external medium. This improves thelight extraction efficiency.

FIG. 20 is a cross sectional view of still another display panel 204.The display panel 203 mentioned above includes the reflective surface213 a on the bank 213 surrounding (enclosing) the light-emitting layer214 and this reflective surface 213 a is disposed on the side of thelight-emitting layer 214. In other words, there is no gap in a verticaldirection or in the direction of emitting between the reflective surface213 a and the light-emitting layer 214. Thus, the leakage of light(cross talk) emitted from the light-emitting layer 214 is prevented andthe efficiency of light is the maximum in this structure. Moreover, asmall portion of the protective layer 218 is deposited on the bank 213,but there may be the protective layer 218 on the anode layer 215. It isalso possible to have a structure in which the anode layer is covered(enclosed) by the protective layer 218 and the thickness of theprotective layer is either same as the height of the bank 213 or lessthan the height of the bank 213. This enables to make a very thindisplay panel. However, in this display panel 203, there is a need toform the insulating layer while forming the reflective surface 213 a, toavoid short circuit between the electrodes. This may complicate themanufacturing of the display panel.

To avoid this, the display panel 204 in FIG. 20 includes a protrusion325 instead of forming the reflecting film 224 on the inclined surface213 a of the bank 213. The protrusion 325 is made of an insulatingmaterial and is formed on a side of the protective layer 218 in aposition overlapping with the bank 213. The reflecting film 324 isdeposited on an inclined surface 325 a of the bank and an area that issurrounded by the protrusion 325 is covered by the protective layer 218.With this display panel 204, there is no concern of short-circuit ofelectrodes due to the reflecting film and there is no need to form aninsulating film to prevent the short circuit between the electrodes.Thus, the display panel 204 can be manufactured with ease. However,there is a gap between the light-emitting layer 214 and the reflectivesurface 325 a in the direction of emission due to which there is apossibility of leakage of light in the bank 213. The height of theprotrusion 325 in the display panel 204 can be made to be the about thesame as that of the bank 213, but it is desirable to reduce the heightof the protrusion 325 to make the display panel thin. On the other hand,when there is need to have some thickness to perform the function of theprotective layer 218 sufficiently, the thickness of the protective layer218 can be controlled freely by adjusting the height of the protrusion325.

Furthermore, in this embodiment also, similar to the display panel 203,the refractive index of the protective layer 218 that is an outputlayer, is either approximately same or greater than that of thelight-emitting layer 214. Due to this, there is no component in theoutput light reflected in the direction D1 (direction of emission orforward direction) of the user 209. Consequently, the whole of theoutput light from the light-emitting layer 214 is transmitted to theprotective layer 218 that is an output layer.

In the display panel mentioned above, the reflective layer, which allowsthe light that has undergone total reflection at an interface to bereflected and guides the light to the angle changer, is the cathodelayer being one of electrodes. However, the reflective layer and thecathode layer may be distinguished from each other. FIG. 21 and FIG. 22are cross sectional views of display panels in this case. As shown inFIG. 21, a reflective layer 302 a may be disposed between thelight-emitting layer 14 and a cathode layer 303 a. This reflective layer302 a must have sufficient conductivity so that a voltage is applied tothe light-emitting layer 14 by the two electrodes (the anode layer 15and the cathode layer 303 a). On the other hand, as shown in FIG. 22, areflective layer 302 b may be disposed immediately below a cathode layer303 b. This cathode layer 303 b must have sufficient transparency likethe anode layer 15.

The display panel according to the present invention is explained withan example of the display panel that is used in a portable telephone.However, the present invention can also be applied to a small sizeddisplay panel that is used in personal digital assistants (PDA), carnavigation etc. The present invention can also be applied to a displayfor a personal computer, a television, a car navigation system, a bigsized display panel of 30 inches that are being developed a lot inrecent years. Moreover the light-emitting layer that uses an organic ELelement is explained here. However, the present invention can also beapplicable to a PDP, an LED, an inorganic EL element, and organic ELelement, a field emission element etc. that use a display panel in whicha light-emitting layer that emits light when the voltage is appliedbetween the electrodes.

Thus, in the present invention, by utilizing the total reflection at theinterface actively, the angle of the light that is fully reflected ischanged by the angle changer like the reflective surface that isprovided at a periphery of the light-emitting layer. Thus, by employingthe thin protective layer the light extraction efficiency can beimproved similarly as in the conventional display panel that includes areflecting inclined surface. This enables to manufacture the bright andvery thin self-emitting element and the display panel. Moreover, thepresent invention enables to change the angle of propagation of light ata low (height) reflective surface thereby improving not only the lightextraction efficiency but also the brightness of each pixel by reducingthe size of a pixel. Thus, the self-emitting element and the displaypanel, in spite of being thin can display very bright images andcharacters than before.

Furthermore, in the present invention, by making the refractive index ofthe output layer same or greater than that of the light-emittingelement, the whole of the output light from the light-emitting elementcan be input to the output layer. By providing the angle changer in theoutput layer that changes the optical path, the refractive index is madegreater. This enables to output the output light efficiently to theexternal medium to incident it at an angle smaller than the criticalangle with respect to the interface of the external medium and theoutput layer having its critical angle decreased. Consequently, thisenables to improve the efficiency of the light to a great extent that isoutput from the light-emitting element and to provide the control panelthat can display a clear and bright image.

Furthermore, in the present invention, in addition to the employing ofthe thin protective layer, the refractive index of the output layer ismade same as that of the light-emitting element or greater. This enablesto improve further the light extracting efficiency.

1. A light-emitting element comprising: a substrate having a firstsurface; a bank above the first surface of the substrate; a displaylayer disposed surrounding the bank, the display layer including alight-emitting layer; a protective layer disposed above the displaylayer, the protective layer having a second surface facing away from thedisplay layer; a protrusion disposed on the bank, a width of theprotrusion near the bank being larger than a width of the protrusionnear a tip of the protrusion; a reflecting film disposed on an inclinedsurface of the protrusion; the first surface being separated from thesecond surface by a first distance that is smaller than a seconddistance separating the first surface from the tip of the protrusion. 2.The light-emitting element according to claim 1, the display layerincluding: a first electrode layer; a second electrode disposed fartherfrom the substrate than the first electrode; and a light-emitting layerdisposed between the first electrode and the second electrode.
 3. Thelight-emitting element according to claim 1, the first electrode being areflective layer having a function to reflect a part of light emittedfrom the light emitting layer.
 4. The light-emitting element accordingto claim 1, a reflective index of the protective layer being larger thana reflective index of the light-emitting layer.
 5. The light-emittingelement according to claim 1, the protrusion being formed by aninsulating material.
 6. A display panel being formed by disposing theplurality of light-emitting element in a two dimensional in matrixaccording to claim
 1. 7. A display panel being formed by disposing theplurality of light-emitting element in a two dimensional in matrixaccording to claim
 2. 8. A display panel being formed by disposing theplurality of light-emitting element in a two dimensional in matrixaccording to claim
 3. 9. A display panel being formed by disposing theplurality of light-emitting element in a two dimensional in matrixaccording to claim
 4. 10. A display panel being formed by disposing theplurality of light-emitting element in a two dimensional in matrixaccording to claim 5.